1. Field of the Invention
The present invention relates to an optical cell measurement apparatus for measuring spectrum of a substance present in a gas cell.
2. Description of Related Art
For measurement of a spectrum, a plurality of spherical mirrors are disposed in opposed relation in a gas cell, and light is inputted obliquely into the cell thereby to be reflected on the spherical mirrors a plurality of times (twice to 20 times) and then outputted. Thus, the length of a light path is substantially extended. The gas cell having the substantially extended light path is generally referred to as “long light path gas cell.” A measurement method using the long light path gas cell makes it possible to convert a low level spectrum which is generally difficult to measure into a relatively strong signal for measurement of the spectrum (W001/013091).
With the long light path gas cell, however, it is necessary to employ the spherical mirrors and input the light not squarely but obliquely with respect to the spherical mirror in the gas cell. Therefore, a focus position at which vertically polarized light rays are brought to a focus does not coincide with a focus position at which horizontally polarized light rays are brought to a focus due to aberration of the spherical mirror. This tendency is increased as the angle of the obliquely inputted light increases. That is, the tendency becomes more remarkable as a distance between a light input position and a light output position of the gas cell increases.
Therefore, if the light rays polarized in one of the polarization directions are brought to a focus for outputting the light rays inputted into the gas cell, the light rays polarized in the other polarization direction are brought out of a focus. This makes it impossible to output all the light rays, resulting in reduction of the transmittance of the gas cell.
It is therefore an object of the present invention to provide an optical cell measurement apparatus which has a function of correcting the aberration of a spherical mirror disposed in a gas cell thereof to prevent reduction of the transmittance of the gas cell.